Ultra-high molecular weight polyethylene (UHMWPE) is a unique polymer with outstanding properties. It can be compression molded to obtain products which have high abrasion, wear, and fatigue resistance and strength. Also, it can be processed by solution techniques and drawn to form ultra-high modulus and strength multifilament fibers. Specially prepared single crystal morphologies of this polymer can be extruded and drawn to produce fibers with a modulus approaching theoretically predicted values. UHMWPE is intractable by conventional melt processing techniques such as extrusion and injection molding because of its extremely high molecular weight and melt viscosity, and is processed by powder sintering techniques used for ceramics and metals, and by ram extrusion.
Information in the prior art is available separately on the swelling behavior of crosslinked systems in the presence of a solvent, the dissolution of polymers, and the diffusion of solvents into amorphous polymers. However, there is only limited information in the literature on simultaneous kinetics of swelling and dissolution in polymers. Ultra-high molecular weight polymers are unique in the respect that significant swelling can occur without dissolution, even though the polymers are uncrosslinked and are crystalline in nature. This phenomenon occurs because of the long reptation and relaxation times of the molecular chains and the high molecular chain entanglement concentrations in these systems.
There has been extensive investigation on producing high modulus and high strength products from flexible and linear commodity polymers. In Zachariades patents Nos. 5,030,402 and 4,820,466 solid-state deformation processes are disclosed for achieving high modulus products. Smith et al (P. Smith and P. Lemstra, J. Mater. Sci., 1980, Vol. 15, 505 and P. Smith and P. Lemstra British Polymer Journal, 1980, 212) among others reported the gel processing route for the manufacture of high stiffness and strength ultra-high molecular weight polyethylene fibers. This process required the polymer to be dissolved in a solvent, extruded, quenched, freed of the solvent and then subsequently hot drawn. In U.S. Pat. No. 4,413,110, Kavesh et al also described gel spinning from a solution to make ultra-high modulus and strength polyethylenes using 2-5% UHMWPE in paraffin oil. Such a process uses too much solvent making processing difficult. Tapes made from melt crystallizing UHMWPE exhibit draw ratios of only about 8, resulting in final properties of Young's modulus about 1-2 GPa and tensile strength of 0.1-0.3 GPa.
Zachariades in his earlier patent, 4,655,769, differentiated between pseudo-gels and true gels used by other researchers, and described a process for making ultra-high polyethylene tubular products employing pseudo-gel states covering the following salient step points:
Dissolving a starting material of UHMWPE powder in non-volatile solvent at 140.degree.-170.degree. C. to produce a solution; cooling the solution to 123.degree. C. to prepare a pseudo-gel in sheet form; extracting non-volatile solvent by a volatile solvent; compressing the pseudo-gel at 123.degree. C. to form a thin gel-like film which is wrapped around a mandrel; evaporating the volatile solvent while the pseudo-gel film is wrapped on the mandrel; and then drawing the tubular product at around 5.times. at 135.degree. C.
Also, others have attempted to develop polymer products with high strength characteristics. In Sano et al U.S. Pat. Nos. 4,879,076, 5,026,511, 4,996,011, 4,760,120 and 5,002,714, a selected polyethylene obtained using a specific catalyst was drawn at temperatures lower than the polymer melting point, in order to make high modulus and high strength fibers or films. In some cases this specific polyethylene was compression molded, immersed in solvent, solid-phase extruded or rolled, and finally drawn. Their process is similar to an earlier process, (See 1. M. P. C. Watts, A. E. Zachariades and R. S. Porter, "New Methods of Production of Highly Oriented Polymers" in "Contemporary Topics in Polymer Science" Ed: M. Shen, Plenum Press, 1979, p. 297-318 and A. E. Zachariades U.S. Pat. No. 4,820,466). Kobayashi et al in their U.S. Pat. Nos. 5,106,555 and 5,200,129 describe a process for continuous production of polyolefin material by feeding the powder between a pair of belts under compression rolling and stretching the compression-molded olefin.
Mackley and Solbai in their published paper (Mackley, Malcolm R., and Solbai, Somad, "Swell Drawing: A new method of manufacturing high performance polyethylene structures", in Polymer J., 1987, Vol. 28, 1115-1120) present a process of swell drawing to manufacture high modulus and high strength ultra-high molecular weight polyethylene (UHMWPE) tapes. Their process includes the following steps:
1. Preparing a precursor UHMWPE material stock (Hoechst Celanese GUR 415) without orientation using sintering by ram extrusion. Preparation of tapes of thickness 0.010 inch by skiving the UHMWPE stock. PA1 3. Cooling the swollen tape down to room temperature under uncontrolled conditions to crystallize the swollen UHMWPE. PA1 4. Evaporating the volatile solvent from the polymer at 80.degree. C. PA1 5. Drawing the dried tape under isothermal conditions at 4 inch/min (100 mm/min) in the temperature range of 90.degree.-120.degree. C.
2. Swelling the skived tape in decalin or xylene at 100.degree.-130.degree. C. for 1 to 10 minutes to an extent that the weight of the solvent to weight of the polymer Ws/Wp, was up to 20.